I sent mine in for calibration for the second time in 5 years because I was getting weird readings. They sent me a new one back. Not even close to being under warranty, or even calibrated at the specified intervals.

I've heard too many horror stories of people sending in a Sensorcon unit for calibration and it never working properly again. The display on mine has been flashing 'cal' for years, I just ignore that. I bump test it with a long breath before every use, in which it usually reads 2 ppm. Anything over 0 and I'm not using the gas for diving, period. YMMV